Results of several studies have shown abnormal platelet reactivity
and altered blood flow in children with autism. Following these
findings it has been suggested that platelet and vascular endothelium
activation could be one of the contributing factors
to the development and clinical manifestations of the disorder
[16908745].
Relative to this the following case reports are of particular
interest, both describing cases of inflammation of brain blood
vessels resulting in loss of language and emergence of symptoms
of autism. In both cases administration of nicardipine lead
to recovery of language and behaviour [1373338,
11008286].

PET and SPECT scans in autistic children show a decreased
cerebral blood floow in some regions of the brain [12077922,
10960047]
and cerebral water content was found to be raised in brain grey
matter in children with autism [16924017].
A model has been suggested in which the observed gray matter
abnormality could be inflammatory (see Immunity-Inflammation).
This finding of celebral edema at the same
time offered an alternative explanation for enlarged brain size
in autism, which up to then had been hypothesised to be due
to lack ‘pruning’ of neurons during development.

For many vessels, including cerebral arteries, calcium entry
through LTCC constitutes the main fraction of contractile calcium.
This is particularly true for immature cerebral arteries, which
are totally dependent on calcium influx through LTCC
channels for contraction, due to relative lack of intercellular
calcium stores, and in which the expression of these channels
is twice as high as in the adult arteries [11742831].
One of possible mechanisms through which excessive calcium influx
via LTCC could be causing restricted blood flow is through its
effect on synthesis rate of endothelin-1, a potent vasoconstrictor
in microvascular endothelial cells [12388093].
Abnormal regulation of L-type calcium channels is directly responsible
for abnormal proliferative responses in vascular smooth muscle
in various forms of cerebral arteriolar injury associated with
endothelial dysfunction [14724353].

Following
these findings, a scenario can be suggested in which disturbances
in the functioning of LTCC can easily lead to vasoconstriction
and decreased cerebral blood flow, as observed in autism.

In an experimental animal model of hydrocephalus and chronic
cerebral ischemia, protective effect against declines in motor
and cognitive behavior exerted by nimodipine, a LTCC blocker,
was thought to be most likely based on improved blood flow [11354411].
Several other calcium channel antagonists have shown various
degrees of neuroprotection through improvement of cerebral blood
circulation [9877076].

In relation to increased water content in the brain (brain edema),
one critical event in its development is breakdown of tight
endothelial junctions which make up the blood-brain
barrier (BBB), which allows fluid to penetrate into brain. Calcium
plays a major role in endothelial junctions, whose function
is necessary for the barrier characteristics of cerebral microvessels.
G-proteins and several calcium linked proteins and enzymes also
seem to be closely involved in junction formation and maintenance
[12053015,
1920385].
Calcium ions could therefore alter BBB junction integrity through
various signalling cascades, as well as through direct interaction
with junction proteins. Regulation of extracellular and intracellular
calcium levels seems to be critical in the normal functioning
of the BBB.

Several proinflammatory mediators, including various cytokines
and chemokines, have direct and indirect effects on the BBB
leading to BBB disruption [16671502].
Humal endothelial cells express functional chemokine receptors
[9461627,
10479649]
that can influence calcium homeostasis via LTCC, leading to
disruption of endothelial cellular function. In particular the
expression of chemokine receptor CCR2 in human
celebral endothelial cells, with its important role
in regulating brain endothelial permeability, has been
uncovered recently [16192992].
Its ligand, monocyte chemoattractant protein-1 (MCP-1) may cause
permeability changes human vascular endothelium cells, possibly
through reduced tight junctions of vascular endothelium cells
[17098977].
Of particular interest should be also the expression of CCR2
receptors in fetal astrocytes, which together with endothelial
cells form BBB, and the interactions of these receptors with
MCP-1 and calcium signalling [12271471,
15689955].
Disruption of chemokine receptor CCR2 abolished both CNS inflammation
and encephalopathy in a murine study [12486156]
(also see Immunity-Inflammation
and Viruses for more details
on chemokine receptors and calcium signalling and findings of
excessive levels of MCP-1 in brain and CSF in autism).

These chemokine receptors can be activated by viral proteins,
for example the chemokine-like protein from human herpesvirus
6 was found to cause calcium mobilization through the CCR2 receptor.
It has been suggested that this protein during reactivation
of the virus could perhaps be involved in the pathogenesis of
the CCR2-dependent disease, multiple sclerosis [12554737].
Infection of mouse epithelial cells with the influenza A-type
virus strain strongly induced the expression of CCR2 and CCR5
receptors, followed by a strong monocyte migration [16849492].
In this context it should be mentioned that prenatal influenza
infection has been implicated in the etiology of autism (see
Viruses).

In the case of the widely studied HIV-1 virus, while its Tat
protein is thought to induce polarization of CCR2 receptors
in astrocytes [15578658],
its protein gp120 has been found to compromises blood-brain
barrier integrity by directly altering expression of tight junction
proteins in brain microvascular endothelial cells. The mechanism
of action behind this effect seems to be linked to its activation
of chemokine receptors CCR5, protein kinase C (PKC) pathways
and subsequent release of calcium from intercellular stores
[16685256].

While for both HIV and HCMV, binding of viral glycoproteins
to the cell surface is sufficient to induce a calcium response,
herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) affect calcium
signalling pathways in endothelial cells by triggering release
of calcium from endoplasmic stores via IP3 receptor activation,
with subsequent additional rises in calcium levels due to activation
of IP3-linked voltage gated membrane channels [14568989].

It may be of interest in this context to note the observation
concerning infection level of endothelial cells by human cytomegalovirus
(HCMV) being influenced by level of PKC. In other words stimulation
of this signalling pathway prior to infection results in an
increase of infection by HCMV while its inhibition prevented
virus replication in murine studies [9175259,
16033962].
Similar effects have been observed in respect to Epstein-Barr
Virus (EBV) replication [2155183]
. It should be noted that calcium is one of the activators of
PKC.

In addition to the implications of chemokine-calcium signalling
interactions in the endothelial and immune cells, similar events
have been suggested to take place in neurons as well, whereas
viral protein interference with chemokine receptors is able
to influence downstream calcium-linked events, including activation
of CREB and interference with cell-cycle proteins in neurons
[11880151,
9826729,
12775414].
(see Brain and Viruses).

Apart from viruses and bacteria, several toxic agents
have been shown to directly interfere with calcium homeostasis
in endothelium, thus contributing to their neurotoxic effects.
For example exposure of cerebral vascular smooth muscle to methanol
results in significant elevation in intercellular calcium. This
methanol-induced cerebral vasospasm as a consequence of large
rises in calcium levles is thought to play a central role in
methanol-induced cerebral edema, brain hemorrhage, and cerebral
and retinal infarcts, resulting in severe deficits in brain
blood flow and disturbances of the CNS [10456574].
Similarly, many of the neurotoxic effects of lead are supposed
to be related to its ability to dysregulate calcium signalling
in cerebral arteries, leading to breakdown of BBB. Apart from
its effects on the endothelial cells, another mechanism by which
lead disrupts BBB is by damaging the astrocytes, which together
with endothelium form a functional BBB. This damaging effect
on astrocytes is also at least partly due to dysregulation of
LTCC function by this heavy metal [1671748]
(see Toxins). A similar mechanism
has been hypothesised to be behind the neurotoxic effect of
methylmercury, suggested to be secondary to astrocytic damage
and disruption BBB brought about by the compound [15288515].

Relative to the function of astrocytes in maintenance
of BBB, cerebral blood flow and cerebral edema, the role of
brain aquaporins in regulation of water homeostasis
and the cerebro spinal fluid formation should be mentioned.
Perturbed water flow via brain aquaporins has been implicated
in many neurological diseases [15561405,
15561410]
and the involvement in the brain edema formation of several
aquaporins that are abundantly expressed in astrocytes, at the
blood brain barrier, has been reported recently. Calcium is
known to play a role in expression of aquaporins and the involvement
of LTCC in aquaporin functioning has been implicated in several
studies [11353665]
and calcium signalling and its effects on vasoconstriction by
astrocytes is one of the mechanism for the regulation of cerebral
blood flow [15356633].
Of interest are the increased levels of brain aquaporin 4 in
autism [18435417].

Calcium overload therefore plays an important role in the occurrence
and development of brain water edema. Treatment with nimodipine
can dramatically reduce the damage of acute infectious brain
edema induced by administration of pertussis toxin and thought
to involve the opening of calcium channels in endothelial and
neuronal cells. This effect is thought to be partially due to
the reduction of the disruption of BBB by this calcium antagonist,
although the opposite effect has been observed in acute brain
damage, in which nimodipine treatment intensified edema formation
[12659709,
9868080,
2118717,
3312189].
Benidipine, another calcium channel blocker, is also shown to
restore endothelial function under particular circumstances
[16172001].